Multi-phase electric power is a common method for providing alternating current for power generation, transmission and distribution. For example, in three-phase systems, three circuit conductors transmit three alternating currents of the same frequency and differing phase. For a particular phase, the alternating currents of the other two phases are shifted in time by one-third and two-thirds of a cycle, respectively. The differing in the phases of the three alternating currents enables constant transfer of power to a load.
Multi-phase electric power provides efficient transfer of power to multi-phase motors. In general, a multi-phase motor includes a stator (i.e. stationary portion) and multiple rotors (i.e. rotating portion) or includes multiple stators and a rotor. Multi-phase electrical power applied to a multi-phase motor results in current flow traversing the multiple stators of the motor. The current flow traversing the multiple stators results in a magnetic field, which produces magnetic torque on the rotor. The magnetic torque applied to the rotor results in rotation of the rotor.
Some conventional multi-phase motor implementations use rotor location information received from sensors for generating necessary signals for controlling multi-phase motors. Other conventional multi-phase motor implementations use detection of zero voltage crossing points associated with Back Electro Motive Force (BEMF).
FIGS. 1A-F are cross-sectional illustrations for an example motor 100.
Motor 100 is a multi-phase motor and includes a stator 102 and a rotor 104. Motor 100 operates as a three-phase motor for converting electrical power to mechanical power. For this example, rotor 104 is located interior to stator 102. Rotor 104 rotates within stator 102, with stator 102 being stationary. Rotor 104 includes a magnet 106, a magnet 107, a magnet 108, a magnet 109, a magnet 110 and a magnet 111. Magnets 106, 107, 108, 109, 110 and 111 provide magnetic fields. Stator 102 includes a first leg 112, a second leg 114 and a third leg 116. Legs 112, 114 and 116 provide magnetic fields. The interoperation of magnets 106, 107, 108, 109, 110 and 111 with legs 112, 114 and 116 cause rotor 104 to rotate within stator 102.
For discussion with respect to FIGS. 1A-F, consider rotor 104 rotating in a clockwise direction.
In FIG. 1A, magnet 106 is located with respect to first leg 112 by an angle 120 also denoted as θ. At this time of the revolution of rotor 104, magnet 106 has not yet reached first leg 112.
In FIG. 1B, rotor 104 has rotated such that magnet 106 has passed first leg 112 by angle 120.
In FIG. 1C, magnet 106 is lagging with respect to second leg 114 by angle 120. At this time of the revolution of rotor 104, magnet 106 has not passed second leg 114.
In FIG. 1D, magnet 106 has passed second leg 114 by angle 120.
In FIG. 1E, rotor 104 has rotated such that magnet 106 is lagging with respect to third leg 116 by angle 120. At this time of the revolution of rotor 104, magnet 106 has not passed third leg 116.
In FIG. 1F, magnet 106 has passed third leg 116 by angle 120.
To efficiently drive motor 100, the relative position of rotor 104 with respect to stator 102 should be known. This may be accomplished by monitoring the relative location of a single point on rotor 104 with respect to stator 102. For purposes of discussion, consider a point 118 on rotor 104. Knowing the location of point 118, with respect to stator 102, enables the determination of the location of all other points on rotor 104 with respect to stator 102. Furthermore, determining the location of point 118 with respect to stator 102 may aid in controlling the operation of motor 100. In particular, motor 100 may be driven differently for the configuration of FIG. 1A than for the configuration in any one of FIGS. 1B-F.
FIGS. 2A-F are cross-sectional illustrations for example conventional motor 100 at different times of operation.
For purposes of discussion, consider the configuration of motor 100 of FIG. 2A, wherein rotor 104 is rotating with an angular velocity, as noted by arrow 204, within stator 102. At some time, the polarity of the magnetic field provided by stator 102 should be opposite to that of the magnetic field provided by rotor 104 so as to “pull” rotor 104 toward stator 102. In this example, magnet 106 is arranged so as to provide a negative magnetic field radially outward toward stator 102. At this time, first leg 112 is driven so as to provide a positive magnetic field radially inward toward rotor 104. An attraction, as noted by an arrow 202, results from the opposite magnetic fields presented by magnet 106 and first leg 112. The attraction indicated by arrow 202 induces rotation of rotor 104 at the angular velocity, as noted by arrow 204.
Rotor 104 will continue to rotate in a clockwise direction, as shown in FIG. 2B. Similar to FIG. 2A, in FIG. 2B magnet 106 is still arranged so as to provide a negative magnetic field radially outward toward stator 102. At this time, first leg 112 is still driven so as to provide a positive magnetic field radially inward toward rotor 104. An attraction, from the opposite magnetic fields presented by magnet 106 and first leg 112, is maintained. The maintained attraction maintains rotation of rotor 104 at an angular velocity, as noted by an arrow 206.
At some time, the polarity of the magnetic field provided by stator 102 should be reversed to “push” rotor 104 away from stator 102. As shown in FIG. 2C, the polarity of the magnetic field provided by first leg 112 has switched from a positive magnetic field as described with reference to FIG. 2A-B to a negative magnetic field. In other words in FIG. 2C, magnet 106 is still arranged so as to provide a negative magnetic field radially outward toward stator 102. However, first leg 112 is driven so as to provide a negative magnetic field radially inward toward rotor 104. The similar magnetic fields provided by first leg 112 and magnet 106 create a repulsion, as noted by an arrow 208. Repulsion 208 maintains rotation of rotor 104 at an angular velocity, as noted by an arrow 210.
Rotor 104 will continue to rotate in a clockwise direction, as shown in FIG. 2D. Similar to FIG. 2C, in FIG. 2D magnet 106 is still arranged so as to provide a negative magnetic field radially outward toward stator 102. At this time, first leg 112 is still driven so as to provide a negative magnetic field radially inward toward rotor 104. A repulsion, from the similar magnetic fields presented by magnet 106 and first leg 112, is maintained. The maintained repulsion maintains rotation of rotor 104 at an angular velocity, as noted by an arrow 212.
For proper operation of motor 100, switching the polarity of the magnetic field provided by first leg 112, for example as described above with respect to FIG. 2C, must be performed at an appropriate time, i.e., at the correct relative position of rotor 104 with respect to stator 102. Accordingly, appropriate switching of magnetic field for first leg 112 requires an accurate determination of the location and velocity for rotor 104 with respect to stator 102.
FIG. 2E illustrates an example of improper timing of the switching of the polarity of the magnetic field provided by first leg 112. In FIG. 2E, rotor is rotating in a clockwise direction at an angular velocity, as noted by an arrow 216. First leg 112 is driven to provide a radially inward negative magnetic field. Because first leg 112 is driven in this manner at this time, the radially inward negative magnetic field provided by first leg 112 repels against the radially inward negative magnetic field provided by magnet 106. The repelling similar magnetic fields results in a repulsion, illustrated by arrow 214, between first leg 112 and magnet 106. In this example, the relative location of rotor 104 with respect to stator 102 may have been incorrectly ascertained resulting in the configuration of the magnetic field for first leg 112 being switched at an incorrect point in time, resulting in an unpredictable operation of motor 100
As shown in FIG. 2F the repulsion, illustrated by arrow 214, between first leg 112 and magnet 106 may result in the undesired termination of rotation for rotor 104.
FIGS. 2A-F illustrate the importance of accurately determining the position and velocity of rotor 104 (relative to stator 102) associated with the operation for a three-phase motor. As the accuracy of the relative position and velocity increases, the more efficiently the three-phase motor may be operated.
FIG. 3 illustrates an example cross-section timing selection diagram for determining the position of rotor 104 with respect to stator 102.
For example, a position 302 is the position of point 118 on rotor 104 at time t1 as shown in FIG. 1A, a position 304 is the position of point 118 on rotor 104 at time t2 as shown in FIG. 1B, a position 306 is the position of point 118 on rotor 104 at time t3 as shown in FIG. 1C, a position 308 is the position of point 118 on rotor 104 at time t4 as shown in FIG. 1D, a position 310 is the position of point 118 on rotor 104 at time t5 as shown in FIG. 1E and a position 312 is the position of point 118 on rotor 104 at time t6 as shown in FIG. 1F.
Control of motor 100 may require determining the location of rotor 104 with respect to stator 102, for example by determining the location of point 118, at various points in time in order to efficiently and properly drive motor 100. An incorrect determination for the location of point 118 may result in inefficient and improper operation of motor 100. For example, if driven improperly, the magnetic fields associated with rotor 104 and stator 102 may repel one another as discussed above with reference to FIG. 2E or rotor 104 may cease rotating as discussed above with reference to FIG. 2F.
There are many known systems and methods for determining the position and velocity of a rotor, with reference to a stator, in a multi-phase motor. Many deal with detecting the BEMF. However, to detect the BEMF, additional circuitry is required.
What is needed is a system and method for determining the position and velocity of a rotor, with reference to a stator, without relying on the BEMF.